1. Field of the Invention
This invention relates to an internal frame of a wheeled suitcase and more particularly, to an internal frame comprising a base, a U-shaped frame mounted on top of the base, and a central frame mounted between the U-shaped frame and the base for strengthening the structure of the wheeled suitcase.
2. Description of the Prior Art
Wheeled suitcases popular for traveling because they can easily be pulled rather than carried. In order to reduce the overall weight of a suitcase, most wheeled suitcases use a light-weight internal frame with a layer of flexible material covered on its outside. In such design the internal frame becomes the most important component in supporting the structure of a wheeled suitcase.
One internal frame design is disclosed in U.S. Pat. No. 5,295,565 by Latshaw entitled "Wheeled Luggage". It discloses an internal frame which comprises upper and lower cast skeletal frame members supported by two vertical tubular members in between. The structure of this internal frame is quite simple but it may cause a stress problem over its frame contacting parts. A loaded wheeled suitcase may place heavy stress over the four contacting parts between the two tubular members and the upper and lower frames. In order to make the internal frame strong enough to support the wheeled suitcase the four contacting parts must be made very strong or it may collapse under heavy pressures when traveling. Besides, periodic vibration and/or shock forces exerted over the four contacting parts may also weaken their connections and thus make the internal frame unreliable.
Another internal frame commonly seen in the market is shown in FIG. 1. The internal frame 10 comprises a base 11, a U-shaped frame 12 mounted on top of the base 11, and a central frame 13 mounted between the base 11 and the U-shaped frame 12 for strengthening the structure of the internal frame 10. The central frame 13 is a rigid plastic board for supporting the stuff loaded inside the suitcase when the suitcase is tilted and pulled toward its rear side. There are several problems associated with this internal frame structure. First, the central frame 13 is very bulky and thus may increase the weight of the suitcase. Second, the center and front parts of the base 11 are not supported by the central frame 13 and thus may be bent downward when heavy stuff is loaded in the suitcase. Third, this structure can not take any piggyback suitcase on its upper front end because there is no supporting structure there for taking such suitcase.